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MAKING BETTER DECISIONS WITH O S C S SS SSPROBABILISTIC RISK ASSESSMENT

ByB. John Garrick

Presented at the Workshop onRisk Assessment and Safety Decision Making Under Uncertainty

Hosted byyConsortium for Risk Evaluation with Stakeholder Participation

and Vanderbilt University Bethesda, MDBethesda, MD

September 21-22, 2010

ANATOMY OF A DECISION

PA(c)CA = P

COST, c

BENEFIT, b

PA(b)BA = P

P

UTILITY OF OPTION A:UA = U(<CA, BA, RA>)

•••

DAMAGE, x

RA = ø PA(x)

••OPTION B PRA BAYES’

THEOREM

E1E2•••

COST

CN = P

BN = P

•••POINTOFDECISION

POINTOFDECISION

•EN

BENEFIT

BN P

DAMAGE

RN = ø

UTILITY OF OPTION N:UN = U(<CN, BN, RN>)

OPTIMAL DECISION: MAX (U U ••• U )

PP

Making Better Decisions with PRA B. John Garrick2

DAMAGEOPTIMAL DECISION: MAX (U A, UB, ••• UN)

Road Map


PRA and Decision Analysis

Risk is defined by the “triplet” set of questions. What y p qcan go wrong? How likely is it? What are the consequences?The role of probabilistic risk assessment in the decisionThe role of probabilistic risk assessment in the decision process is to quantify the outcomes of all the options being considered in the decision.PRA is thus the linkage (logic) between the desired probability curves, which are the input to the decision analysis and all the relevant evidence especially theanalysis, and all the relevant evidence, especially the contributors to the risk.


AN EXAMPLE


State Operated Disposal Area (SDA)

• A 15 acre pre-Part 61 near-surface radioactive waste disposal facility in southwestern New York that operated from 1963 1975operated from 1963-1975.

• Received waste from offsite locations and waste generated by the onsite nuclear fuel reprocessing operations.

• Inventory in the trenches included 230 radionuclides having an estimated activity of 128 000 curies decayedhaving an estimated activity of 128,000 curies, decayed to the year 2000.


SDA Aerial View

7

SDA Trenches and Watershed

8

Decision Options

• Exhume all waste and contamination from the SDA.• Close the SDA in place.• Manage the SDA in place for 10 years to allow

dditi l d t ll ti d l i t dd ladditional data collection and analysis to address long-term issues and identify safe and cost effective management or removal options.g p


Requirement of the Decision

A quantitative answer to the question, “What is the risk of operating the SDA for the next 30 years with its current physical and administrative controls.”


The Decision

The QRA results were instrumental in supporting the decision by the New York State Energy Research and Development Authority to manage the SDA in place forDevelopment Authority to manage the SDA in-place for another decade.


Methodology

• Triplet definition of risk.• Based on a structured set of scenarios.• Quantification of uncertainties with probability

distributions.• C dibilit d fi iti f b bilit• Credibility definition of probability.• Data processing rooted in the fundamental rules of

logic.g


Structure of QRA Model


For Each Scenario, Risk Is Determined By

• Frequency of disruptive or natural processes that cause a release of radioactive materials from the disposal area.area.

• Form, quantity, and radionuclide content of the materials released.

• Distribution, dilution, and deposition of the released materials.

• Public exposure and radiation dose• Public exposure and radiation dose.


Radionuclide Release Mechanisms

• Liquid releases via groundwater in the subsurface.• Liquid overflows and surface water releases• Liquid overflows and surface water releases.• Releases of solid and liquid radioactive materials via

physical breaches of the trenches.• Airborne releases via physical disruption of the SDA

site.


Dose Receptors

• A permanent resident farmer on Buttermilk Creek near its confluence with Cattaraugus Creek.

• A transient recreational hiker/hunter who traverses areas along Buttermilk Creek and the lower reaches ofareas along Buttermilk Creek and the lower reaches of Franks Creek.


Highest Risk Scenarios

• Lateral groundwater flows through the ULT: 37%• Physical breaches of the waste trenches: 35%• Lateral groundwater flows through the WLT: 10%• N b d diti 6%• No geomembrane and severe conditions: 6%


Contribution by Release Mechanism

• Groundwater flow through the subsurface: 55%• Trench breaches by erosion, landslides: 36%• Trench overflows and surface water runoff: 9%• Ai b l f h i l i t 0 1%• Airborne releases from physical impacts: <<0.1%


SDA Risk Curves

1.0E-03

1.0E-02

1 0E 05

1.0E-04

t/ y

ear)

5th Percentile

95th Percentile

1.0E-06

1.0E-05

requ

ency

(eve

nt

MedianMean

1.0E-08

1.0E-07

Rel

ease

Fr

1 0E-10

1.0E-09

19

1.0E-101.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05

Dose (mrem in 1 year)

Vertical Slice

The analysts are 90% confident that the frequency of a dose of 100 mrem in 1 year or greater is between 3.9 x 10-4 and 6.4 x 10-4 event per year.p y

Making Better Decisions with PRA B. John Garrick

Uncertainty in Release Frequency for Exceeding a Dose of 100 Mrem in 1 Year, Probability Density Format, y y

bability Den

sity

Pro

1.0E‐04 3.0E‐04 5.0E‐04 7.0E‐04 9.0E‐04 1.1E‐03 1.3E‐03 1.5E‐03

Release Frequency (event / year)

21

Horizontal Slice

The analysts are 90% confident that the frequency range of a 1 x 10-3 event per year or greater results in a dose between 10 and 31 mrem in 1 year.

Making Better Decisions with PRA B. John Garrick

Uncertainty in Dose for Release Frequency of 1.0E-03 Event/Year, Probability Density Format25

1.0E 03 Event/Year, Probability Density Format

20

15

ability Den

sity

10Proba

5

0

0 10 20 30 40 50 60 70 80 90 100

Dose (mrem in 1 year) 23

Conclusion

The QRA results confirm that the public health risk from operating the SDA for the next 30 years is well below widely applied radiation dose limits.


NUCLEAR PLANT EXAMPLE


Nuclear Power Plant Risk Results

Nuclear Power Plant Risk Results (Continued)(Continued)

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